Astronomer

To become an astronauts you have to qualify these basic requirements-
Bachelor's degree from an accredited institution in engineering, biological science, physical science, or mathematics.
At least 1,000 hours pilot-in-command time in jet aircraft. Flight test experience is highly desirable.
Ability to pass a NASA space physical which is similar to a military or civilian flight physical and includes the following specific standards -
Distant visual acuity- 20/100 or better uncorrected, correctable to 20/20 each eye.
Blood pressure- 140/90 measured in a sitting position.
Height between 62 and 75 inches.
Astrophysicists have a high chance of becoming astronauts.
But, if you want to get more detail then please read the given attach files Or you can visit the given officials site -
http://www.nasajobs.nasa.gov/astronauts
http://www.jsc.nasa.gov/Bios

An astronaut is someone who is giving training by the space flight program to work as the pilots.
Qualifying exam
Bachelor's degree in biological Science, Engineering, Mathematics or Physical Science from a recognized university.
They need to obtain any one of the following degrees from renowned institutes like IISC or IIT.
Ph.D (Astrophysics)
Ph.D (Geophysics)
Ph.D (Metaphysics)
After graduation, candidates can appear for the entrance exam which is held by the NASA for the recruitment of scientists, Astronauts etc.
You may also find employment in ISRO
There are many openings in ISRO, which is conducted for the recruitment of scientists, engineers .
In order to join ISRO, you need to clear joint entrance exam by IIT's.
Eligibility to apply:
Should be a citizen of India
You should be at least 21 years of age by the time you join. Age relaxation is there for SC/ST candidates.
Need to be graduate from a recognized university or board.